Constellation Energy (CEG.US) has agreed to acquire minority-owned Calpine Corp. for $1.64 billion, adding dozens of power generation assets across the United States.The two companies announced in a statement on Friday that Constellation would pay in cash and stock. Additionally, Constellation will take on Calpine's roughly $12.7 billion in net debt. Shares of Constellation rose more than 13% before the market opened on Friday.The acquisition will be one of the largest in the U.S. power generation sector, and will provide a significant return for Calpine shareholders Energy Capital Partners, Canada Pension Plan Investments, and Access Industries, who took the company private in 2018 for $5.5 billion.As energy consumption surges in the U.S., Constellation is seeking to expand. Power demand is expected to grow nearly 16% over the next five years, driven by data centers running on artificial intelligence, new factories, and electrification of everything from cars to home heating. All of this has pushed up the value of power providers, with Constellation's stock up more than 90% last year.When Constellation spun out of utility owner Exelon (EXC.US) in 2022, CEO Joseph Dominguez said he was interested in growing the company through acquisitions. The following year, Constellation bought a 44% stake in a nuclear plant in Texas from NRG Energy (NRG.US) for $1.75 billion.But the deal for Calpine dwarfs that in both size and value. Constellation runs natural gas and hydroelectric facilities, as well as the largest nuclear fleet in the U.S., with a total capacity of 32.4 gigawatts. A gigawatt is the equivalent of a traditional nuclear plant, enough to power more than 850,000 American households. Through the Calpine deal, it will add 78 power plants in 22 states, with a capacity of 27 gigawatts.Under the terms of the Friday statement, Constellation is paying 7.9 times expected 2026 earnings.